Recovery of trioxane



United States Patent 3,346,593 RECUVERY 0F TRIOXANE Gerhard Langecker, Cologne-Lindenthal, Germany, assignor to Josef Meissner Kommanditgesellschaft, Cologne-Bayenthal, Germany No Drawing. Filed Mar. 11, 1965, Ser. No. 439,059 Claims priority, application Germany, Mar. 24, 1964, M 60,421 5 Claims. (Cl. 260-340) The present invention relates to a novel method for the recovery of trioxane from aqueous solutions thereof. More particularly, the invention concerns a method for the recovery of trioxane by extraction with a solvent having a boiling point higher than that of trioxane, and recovering the trioxane therefrom by distillation.

The instant invention provides a process for the recovery of trioxane, which is the trimmer of formaldehyde, from aqueous solutions thereof, by extraction with a solvent and subsequent distillation. There have heretofore been employed as solvents for this purpose, those having boiling points below that of the trioxane, so that in the subsequent processing by distillation, the entire amount of solvent had to be distilled off, which required a 'disproportionately high consumption of energy. Moreover, the use of such lower boiling point solvents has the result that in such processes the trioxane remaining in the sump is very strongly adversely afiected by heat.

It is, therefore, the primary object of the present invention to provide a process which overcomes the above drawbacks, and with this object in view, the present invention resides in a process wherein a solvent is used which has a boiling point substantially higher than that of trioxane. More particularly, in accordance with the invention, a dichlorobenzene is employed for the extraction of the trioxane from its aqueous solution, and upon subsequent distillation the trioxane is removed at the top of the distillation column.

Although other solvents exist which have a boiling point substantially above that of trioxane, it has been found that the dichlorobenzenes are especially suitable for this purpose because their boiling points lie in a favorable range. On the other hand, a solvent having a lower boiling point than that of the dichlorobenzenes would, upon subsequent distillation, leave a greater residue and require a greater reflux ratio. A higher boiling solvent would require considerably greater steam pressure and a larger distillation apparatus.

The following table shows the boiling points of the various dichlorobenzenes in comparison with that of trioxane.

The boiling point of s-trichlorobenzene, for example, is 219 C., so that this solvent would have only limited usefulness.

Of the various dichlorobenzenes, o-dichlorobenzene is particularly suitable and is preferred; however, m-dichlorobenzene is also suitable. The p-dichlorobenzene is advantageously used mainly in admixture with one or both of the other isomers. A mixture of oand m-dichlorobenzene can also be used.

In the practice of the invention the trioxane can be extracted from its aqueous solutions in a Wide range of concentrations, from as low as 0.4% by weight up to the limit of solubility. The ratio of dich-lorobenzene to trioxane solution will depend upon the concentration of the latter, but in general an extraction ratio of 1:1 to 1:15 is employed.

The manner of carrying out the extraction and recovering the trioxane by distillation will be apparent from the examples below, in each of which a solvent having a boiling point higher than trioxane is employed. In these examples various high boiling point solvents were employed in order to establish that the dichlorobenzenes which form the subject of the invention are by far the most suitable. Of the dichlorobenzenes only the o-dichlorobenzene was employed, it being considered as representative of the other two (m and p-)isomers. In the comparison of the various solvents, it will be apparent that mono-chlorobenzene in many respects is the equal of o-dichlorobenzene, but will also be apparent that the dichlorobenzenes are in some respects, particularly those which are essential to the manufacture of trioxane, even more advantageous than mono-chlorobenzene and without some of its disadvantages. This is shown by a comparison of the relative boiling points involved:

TABLE 11 Compound: Boiling point, C. Trioxane 1 15 Chlorobenzene 13-2 Dichlorobenzenes 172-179 Trichlorobenzenes 208-218 The 'following examples illustrate the practice of the invention, but are not to be considered as limiting:

EXAMPLE 1 Extraction with mono-chlorobenzene Through a pulsation type extraction column maintained at 60 C. there were introduced 4 kg. per hour of a 28% by weight aqueous solution of trioxane and 4.45 kg. per hour of chlorobenzene. This corresponds to an extraction ratio of 1:14 There was recovered as the lighter phase 2.9 kg. per hour of a 0.4% by weight aqueous trioxane solution, while the heavier phase comprised 5.55 kg. of a 20.1 weight percent solution of trioxane in chlorobenzene which contained 0.42 weight percent of water, i.e. 2.1 parts water per parts trioxane.

EXAMPLE 2 Extraction with o-dichlorob'en zene Employing the same extraction apparatus and operating conditions described in Example 1, o-dichlorobenzene was used as the extraction agent. With the feed of 5.6 kg. per hour of odichlorobenzene, corresponding to an extraction ratio of 1:1.5, there was obtained an aqueous extracted phase containing 0.4 weight percent of trioxane. 'The heavier organic phase contained 16.7 weight percent trioxane and 0.24 Weight percent water, i.e. 1.4 parts water per 100 parts trioxane.

Tests with trichlorobenzene according to the above described procedure have shownthat no substantially complete extraction of the trioxane from the aqueous solution is possible because of the coefficient of partition of trichlorobenzene with respect to trioxane, which is less than 1. Thus, despite its high boiling point, trichlorobenzene is unsuitable for trioxane extraction. Therefore, only mono-chlorobenzene and dichlorobenzene are comparable. From the foregoing example, it is apparent that the use of chlorobenzene results in a higher concentration of trioxane in the solvent than with dichlorobenzene. This is doubtless a considerable advantage so that it would seem desirable to employ chlorobenzene as the solvent.

Experience has shown, however, that in practice the proportion of water in the organic phase is a major factor in the attainable trioxane concentration. Therefore, the

choice of dichlorobenzenes, although surprising, is nevertheless the'most suitable for actual practice, because although the chlorobenzene solution contains one and a fifth times as much trioxane as the dichlorobenzene solution, it has the disadvantage of containing 1.75 times the amount of water.' This disadvantage of chlorobenzene as a solvent is particularly noticeable in the final distillative separation of the water from the crude trioxane, since with the chlorobenzene the considerably higher water content causes some of the 1.5 amount of trioxane to combine with the water to form an azeotrope which must be recycled for further processing. Furthermore, the boiling point of chlorobenzene, while higher than that of trioxane, is nevertheless so close to it that special expense would be involved in order to take advantage of this slight advance in temperature. In contrast thereto, the boiling point of the dichlorobenzenes is so far above that of trioxane that the advance can easily be technically controlled.

Tests have shown that the novel method of the invention is useful for the separation of crude trioxane from the dichlorobenzene solution, employing therefor a distillation column With five separation and 12 reflux plates, and a reflux ratio of 1:2. In this way a head product is obtained comprising pure trioxane containing less than 0.05 weight percent dichlorobenzene, and a solvent re covery practically free from trioxane.

In addition to the numerous advantages mentioned previously, the method of the invention using dichlorobenzenes as solvents for the extraction of aqueous solution of trioxane represents a remarkable technical advance.

As mentioned previously, o-dichlorobenzene is interchangeable with mand p-dichlorobenzene so that mixtures of all three can also be employed as extraction solvents with substantially the same efiiciency as the individual isomers.

It will be understood that the above description of the present invention is susceptible to various modifications, changes, and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.

What is claimed is:

1. Method for the recovery of trioxane from aqueous solutions thereof which comprises treating an aqueous solution of trioxane with dichlorobenzene, separating the dichlorobenzene extract, and distilling the extract to recover trioxane therefrom.

2. Method of claim 1 in which the extraction solvent is o-dichlorobenzene.

3. Method of claim 1 in which the extraction solvent is m-dichlorobenzene.

4. Method of claim 1 in which the extraction solvent is p-dichlorobenzene.

5. Method of claim 1 in which the extraction solvent is a mixture of dichlorobenzenes.

References Cited UNITED STATES PATENTS 2,347,447 4/1944 Walker 260-340 3,149,127 9/1964 Platz 260340 3,201,419 8/1965 Sennewold et al. 260-340 WALTER A. MODANCE, Primary Examiner,

N. MILESTONE, Assistant Examiner. 

1. METHOD FOR THE RECOVERY OF TRIOXANE FROM AQUEOUS SOLUTIONS THEREOF WHICH COMPRISES TREATING AN AQUEOUS SOLUTION OF TRIOXANE WITH DICHLOROBENZENE, SEPARATING THE DICHLOROBENZENE EXTRACT, AND DISTILLING THE EXTRACT TO RECOVER TRIOXANE THEREFROM. 